Most of the world's English language teachers speak English as a second or third language rather than as their first language. For many, their level of proficiency in English may not reach benchmarks established by their employers, raising the issue that is the focus of this article, namely, what kind of proficiency in English is necessary to be an effective teacher of English? The article seeks to provide an overview of how the role of language proficiency issue has been addressed in the ELT literature. It describes the kind of specialized language skills needed to teach English through English, explores the relationship between language proficiency and teaching ability, considers the impact of language ability on different dimensions of teaching, and raises the implications for language assessment and for the design of language enhancement programmes for language teachers.